The invention relates to a power steering apparatus which may be utilized to reduce the magnitude of a force which must be applied to steering means such as a steering wheel in order to steer a vehicle, and in particular, to an electrically driven power steering apparatus including an electric drive mechanism having an electric motor and which detects the magnitude of a torque that is applied from steering means to a direction controlling mechanism which establishes a particular direction in which the vehicle is to run and which applies a driving force of a magnitude corresponding to the magnitude of the torque to the direction controlling mechanism from the electric drive mechanism.
When it is desired to redirect wheels, a force of an increased magnitude is required to turn a steering wheel when the vehicle is at rest or is running at a low speed. In particular, with FF cars which are increasing in number recently and which have their front wheels designed as driving wheels, a further increase in the steering force is required.
To accommodate for this, a power steering apparatus which assists a driver in a steering operation has been proposed. Such apparatus produces a drive force in accordance with a steering force of a driver, and transmits it to a direction controlling mechanism (hereafter referred to as a steering system) which establishes a particular direction in which the vehicle is to run. Almost all of power steering apparatus which is currently in practical use is of hydraulic type. Thus, such apparatus is provided with a control valve, hydraulic cylinder and the like, and operates to produce an assisting steering force through a movement of a pressure oil in accordance with the steering force. However, it will be noted that such control valve, hydraulic cylinder and the like are bulky in size, and must be connected together through pipings which can only be bent with radii of curvature greater than a given value in order to avoid pressure losses. A hydraulic power steering apparatus must be provided with a reliable oil seal against leakage, which requires a troublesome mounting operation. Thus, a mounting of the power steering apparatus presents a problem in a vehicle such as FF car where available remaining space is small.
To overcome the described problem, there has been proposed an electrically driven power steering apparatus which utilizes an electric motor as a drive source and which detects the magnitude of a torque applied to a steering system from steering means so that the motor applies an assisting steering force to the steering system in a manner corresponding to the magnitude of torque detected. With this arrangement, the space utility is improved, and in addition, a varying magnitude of assisting steering force which cannot be obtained with a conventional hydraulic power steering apparatus, for example, an assisting steering force which is dependent on a vehicle speed, may be developed when used in combination with an electronic controller.
It is to be noted that in an electrically driven power steering apparatus of the kind described, a reliability of a very high level is required in the accuracy with which detector means detects the magnitude of a torque applied to the steering system from the steering means. Thus, if the torque detector means delivers an abnormal signal, a corresponding assisting steering force, which is abnormal, will be applied to the steering system. Specifically, if the torque detector means malfunctions in developing a detection signal as a result of a temperature rise within a running vehicle even though a driver of the vehicle performs no steering operation, a corresponding assisting steering force will be applied to the steering system to cause the running direction of the vehicle to be changed independently from the intent of the driver (hereafter referred to as "auto-steer"). The possibility for the occurrence of such situation could be minimized by providing a plurality of torque detector means to improve the reliability of detection. However, this presents an increased cost as another aspect. In addition, any improved reliability in the detection cannot be perfectly free from the occurrence of an abnormality.
In the event the motor has locked for some reason, as by an overload on the motor which causes a burn-out thereof, it is possible that the driver cannot change the running direction of the vehicle even though he attempts to steer the vehicle. Such occurrence may be considered as equivalent to the application of a force from the electric drive mechanism which tends to block the steering by the driver. In either instance, there is a likelihood of a serious risk during the running of the vehicle, and the risk will be greater with a faster running speed.